add legacy ftime function and sys/timeb.h

despite being marked legacy, this was specified by SUSv3 as part of
the XSI option; only the most recent version of the standard dropped
it. reportedly there's actual code using it.

src/time/ftime.c

@@ -0,0 +1,12 @@
+#include <sys/timeb.h>
+#include <time.h>
+
+int ftime(struct timeb *tp)
+{
+	struct timespec ts;
+	clock_gettime(CLOCK_REALTIME, &ts);
+	tp->time = ts.tv_sec;
+	tp->millitm = ts.tv_nsec / 1000000;
+	tp->timezone = tp->dstflag = 0;
+	return 0;
+}